1. Field of the Invention
The present invention relates to a MEMS (Micro-Electro-Mechanical System) switch that can be applied also as a high frequency circuit switch.
2. Related Background Art
It is desired to apply MEMS, to which a semiconductor process is applied, to various fields. In, for example, a field to which a high frequency circuit is applied, application of MEMS acting as an RF switch is strongly desired.
A MEMS switch applied to high frequency circuit can be used for from direct current (DC) circuit to high frequency circuit and is roughly divided into a DC contact type MEMS switch for supplying power by ohmic contact of two contact points and a capacitive type MEMS switch in which two contact points come into contact with each other through a dielectric film and which can be mainly used only for high frequency of 10 GHz or more.
Since consumer wireless equipment mainly uses a frequency range from about 500 MHz to 5 GHz, the DC contact type MEMS switch in particular has a high value of usage.
An electrostatic drive mechanism is mainly used as a drive mechanism of a conventional DC contact type MEMS switch. This is because a material and a structure are simple and a process is easy.
A typical structure of the MEMS switch is such that a pull-down electrode covered with a dielectric film and an ohmic contact electrode are separately formed in adjacent regions on a substrate as well as a conductive movable beam, which is supported by the base of one side end, is installed so that the other side end thereof is located above the pull-down electrode and the contact electrode. The movable beam has a small spring.
In operation of the MEMS switch, it is opened and closed in such a manner that a voltage is applied between the pull-down electrode and the movable beam, the movable beam is attracted by electrostatic force, and the contact electrode located adjacent to the pull-down electrode is caused to come into ohmic contact with the movable beam.
In the above structure, when an about 10 μm thick movable beam formed by a bulk micro machining method is used, since the movable beam has approximately sufficient rigidity, it is possible to obtain a sufficient amount of press force of the movable beam to the contact electrode.
However, when the movable beam is formed by the bulk micro machining method, a problem arises in that the manufacturing process of the movable beam is made complex and a manufacturing cost becomes expensive.
In contrast, when the movable beam is formed by laminating thin films by a surface micro machining method, manufacturing steps is relatively simple. However, since the thickness of the movable beam is several microns at the maximum and ordinarily 1 to 2 μm and the movable beam is liable to flex due to its small rigidity, it is difficult for the movable beam to apply a sufficient amount of contact pressure to the contact electrode.
Although the bulk micro machining method and the surface micro machining method have advantages and disadvantages, respectively, as described above, it is desirable to employ the surface micro machining method from the view point of suppressing manufacturing cost.
Accordingly, there have been proposed structures for obtaining a sufficient amount of contact pressure force between a contact electrode and a movable beam while employing the surface micro machining method. Refer to, for example, a reference “Inline Capacitive and DC-Contact MEMS Shunt Switches”, Jeremy B. Muldavin and Gabriel M. Rebeiz, IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 11, No. 8, pp. 334-386, AUGUST 2001.
The MEMS switch disclosed in the reference has a structure in which a lattice-like contact electrode is formed on a pull-down electrode, and when a voltage is applied between the pull-down electrode, which is exposed to the opening of the lattice-like contact electrode, and a conductive movable beam, the movable beam is attracted and caused to come into ohmic contact with the lattice-like contact electrode.
In the MEMS switch of the reference, however, since the lattice-like contact electrode is formed on the pull-down electrode, the distance between the pull-down electrode and the movable beam must be set in consideration of the thickness of the contact electrode, and it is almost impossible to adjust the distance. Since the distance between the pull-down electrode and the movable beam is increased by the thickness of the contact electrode, the attraction force between the pull-down electrode and the movable beam is still insufficient, and thus the contact pressure force obtained between the movable beam and the contact electrode cannot be sufficient.
To obtain a larger amount of attraction force between the pull-down electrode and the movable beam, it is necessary to increase the exposed area of the pull-down electrode, that is, to reduce the sizes of the respective portions of the lattice-like contact electrode. However, a problem arises from the arrangement in that the contact resistance between the movable beam and the pull-down electrode is increased.
Further, when it intended to obtain a sufficient amount of attraction force by increasing the voltage applied between the pull-down electrode and the movable beam, another problem arises in that power consumption is increased.
From what has been described above, there is required a MEMS switch having such a structure that even if a movable beam is formed of a deposited thin-film, a sufficient amount of contact pressure force can be obtained between the movable beam and a contact electrode as well as the contact resistance between them can be suppressed small.